Use libtool on FreeBSD

[lantw44]

g-ir-scanner does not use libtool on FreeBSD by default, so we need to add --libtool option to tell g-ir-scanner the correct libtool to use. If we don't have the --libtool option, g-ir-scanner will run the compiler directly, which does not know the .la file.

Patch: http://paste.fedoraproject.org/77471/24453711/